1. Field of the Invention
The invention relates to a resolver/digital converter that converts a signal output from a resolver to angular data.
2. Discussion of Background
As one of devices that detect a rotation angle of a detection target, there are a resolver that outputs a voltage signal that corresponds to a rotation angle of a detection target and a resolver/digital converter that converts the signal output from the resolver (resolver signal) into angular data.
For example, when the resolver is configured to have one-phase excitation and two-phase outputs, if the resolver receives an excitation signal formed of an alternating-current voltage, the resolver outputs two-phase resolver signals that have phases that are different from each other by 90° and that change in accordance with the rotation angle of the detection target. Specifically, when the resolver receives an excitation signal Vex as shown in FIG. 10A, the resolver outputs a sinusoidal first-phase resolver signal Va as shown in FIG. 10B and a sinusoidal second-phase resolver signal Vb as shown in FIG. 10C. When the excitation signal Vex is V0·sin(ωt), the first-phase resolver signal Va is K·V0·sin θ·sin(ωt), and the second-phase resolver signal Vb is) K·V0·sin(θ+90°)·sin(ωt). Here, “ω” denotes an excitation angular frequency, “t” denotes time, “V0” denotes the amplitude of the excitation signal, “K” denotes the transformation ratio of the resolver, and “θ” denotes the rotation angle (electric angle) of the detection target.
The resolver/digital converter captures the two-phase resolver signals Va, Vb, and acquires the amplitudes of the two-phase resolver signals Va, Vb. That is, the resolver acquires a value (K·V0·sin θ) on the bold line in FIG. 10B for the first-phase resolver signal Va, and acquires a value (K·V0·sin(θ+90°)) on the bold line in FIG. 10C for the second-phase resolver signal Vb. The resolver/digital converter computes the rotation angle of the detection target by computing the arctangent of the amplitude of each of the two-phase resolver signals Va, Vb. The resolver/digital converter computes the rotation angle at intervals of a predetermined period, and converts the two-phase resolver signals Va, Vb into digital angular data. As a resolver/digital converter of this type, a device described in Japanese Patent Application Publication No. 2003-315097 (JP 2003-315097 A) has been known.
In the resolver/digital converter described in JP 2003-315097 A, a computation period of the rotation angle is set on the basis of the least common multiple of an excitation period of an excitation signal and a capturing period (analog/digital (A/D) sampling period) of a resolver signal. Specifically, as shown in FIG. 11A, when an excitation period T1 is 80 μs and an A/D sampling period T2 is 50 μs, a computation period T3 is set to 400 μs. Thus, by superimposing data that are captured during the computation period T3, data Ds0 to Ds7 at eight points within one period of the excitation signal are captured as shown in FIG. 11B. The resolver/digital converter described in JP 2003-315097 A determines the amplitude of the resolver signal such that the residual sum of squares between the data Ds0 to Ds7 and a model function of the resolver signal becomes minimum, that is, computes the amplitude of the resolver signal by utilizing a so-called method of least squares. Thus, it is possible to accurately detect the amplitude of the resolver signal. Therefore, the accuracy of detection of the rotation angle improves.
According to a technique described in JP 2003-315097 A, depending on the excitation period and the A/D sampling period, the least common multiple of those periods may be increased and the computation period may become longer. Therefore, a resolver/digital converter that is able to compute a rotation angle within a shorter period has been demanded.